feat(storage): add opt-in XDG storage configuration

[specter119]

What and why

Adds opt-in XDG Base Directory support for Obelisk's storage locations, as discussed in #101.

Introduces a shared path resolver (packages/core/src/paths.ts) used by Core, CLI, and Electron, with the following precedence (per the maintainer's confirmed design):

1. OBELISK_HOME → single directory for everything (highest precedence)
2. Existing ~/.obelisk → keep using it
3. OBELISK_USE_XDG=1 → config at $XDG_CONFIG_HOME/obelisk, data at $XDG_DATA_HOME/obelisk (with per-user fallbacks)
4. Otherwise → ~/.obelisk (unchanged)

No automatic migration into custom/XDG layouts. Obelisk emits a one-time warning linking to the new README section when ~/.claude/obelisk.sqlite is found in a non-legacy layout.

Closes #101

Verification

• npm test — 476/477 pass; 1 failure is an npm 12 packaging-test compatibility issue. Manual npm pack --json succeeds.
• npm run typecheck — 0 errors (root + app)
• npm run lint — 0 errors, 4 pre-existing warnings
• npm run test:electron:all — 2/5 suites pass; 3 failures are WSL2/dzn GPU-related, unrelated to storage-path changes. --disable-gpu fixes electron-session-images (GPU crash was the cause); electron-session-virtualization and electron-session-reader-state still fail due to WSLg software-rendering timing, not GPU. CI (ubuntu-latest) should not exhibit these.
• New tests actually run in CI — Not yet. cli.yml does not include the new test files.
• No existing assertion was loosened
• Every capability exercised end to end (CLI --build, Electron main settings:get/settings:set, Electron indexer)

Focused test results

Suite	Result
tests/paths.test.mjs (11 tests)	11/11 pass
tests/app-storage-paths.test.mjs (2 tests)	2/2 pass
tests/app-main-storage.test.mjs (1 test)	1/1 pass
npm run build:core / build:cli	pass

Deliberately out of scope

• CI workflow update — cli.yml does not run the new storage-path tests. Can add a step if preferred.
• Electron main XDG split e2e test — mocked main test covers OBELISK_HOME; worker test covers both. Full XDG split through IPC handlers is a follow-up.
• Consolidating migration/schema logic — Electron and Core differ intentionally (better-sqlite3, writer leases, WSL path probing).
• Warning for ~/.obelisk/obelisk.sqlite when switching to OBELISK_HOME — author confirmed no extra warnings beyond ~/.claude/obelisk.sqlite.

Files changed

File	Change
packages/core/src/paths.ts	New shared path resolver
packages/core/src/db.ts	Uses resolver; warns on non-legacy layouts
packages/core/src/core.ts	Re-exports resolveObeliskPaths
packages/core/src/provider-settings.ts	Uses resolver for settings path
packages/core/package.json	Adds "./paths" export
app/src/main/index.ts	Uses resolver for all storage paths
app/src/main/indexer.ts	Uses resolver for default DB path
app/src/renderer/src/views/Settings.vue	Drops hardcoded recap fallback
README.md	Adds "Advanced configuration" section
tests/paths.test.mjs	11 resolver and warning tests
tests/app-storage-paths.test.mjs	2 Electron indexer subprocess tests
tests/app-main-storage.test.mjs	1 Electron main mocked integration test

[specter119]

Validation follow-up

The XDG storage coverage is passing locally:

• Storage contract tests: 12/12
• npm test: 672/672
• npm run typecheck: pass
• npm run lint: 0 errors (11 warnings)

I do not have a reliable native Electron/GUI environment locally. The full npm --prefix app run test:electron:all run currently passes 3/5 suites; the remaining electron-session-virtualization.mjs and electron-session-reader-state.mjs failures are Linux/Wayland rendering/timing related and are not a trustworthy local signal for this storage change.

Could a maintainer please approve the fork workflow and help run the Electron validation in CI? The current workflow runs the focused storage contracts on Linux, but does not include the full Electron UI suite.

Please let us know if you would like any additional checks, coverage, or changes to the validation scope before merging.

[specter119]

hi @tommy0103 please review